Re: [PATCH v2 5/5] powerpc/lib: inline memcmp() for small constant sizes

Segher Boessenkool
2018-05-17T13:56:37Z

On Thu, May 17, 2018 at 12:49:58PM +0200, Christophe Leroy wrote:
> In my 8xx configuration, I get 208 calls to memcmp()
> Within those 208 calls, about half of them have constant sizes,
> 46 have a size of 8, 17 have a size of 16, only a few have a
> size over 16. Other fixed sizes are mostly 4, 6 and 10.
>
> This patch inlines calls to memcmp() when size
> is constant and lower than or equal to 16
>
> In my 8xx configuration, this reduces the number of calls
> to memcmp() from 208 to 123
>
> The following table shows the number of TB timeticks to perform
> a constant size memcmp() before and after the patch depending on
> the size
>
> Before After Improvement
> 01: 7577 5682 25%
> 02: 41668 5682 86%
> 03: 51137 13258 74%
> 04: 45455 5682 87%
> 05: 58713 13258 77%
> 06: 58712 13258 77%
> 07: 68183 20834 70%
> 08: 56819 15153 73%
> 09: 70077 28411 60%
> 10: 70077 28411 60%
> 11: 79546 35986 55%
> 12: 68182 28411 58%
> 13: 81440 35986 55%
> 14: 81440 39774 51%
> 15: 94697 43562 54%
> 16: 79546 37881 52%

Could you show results with a more recent GCC? What version was this?

What is this really measuring? I doubt it takes 7577 (or 5682) timebase
ticks to do a 1-byte memcmp, which is just 3 instructions after all.

Segher